In digital communication systems, it is important to obtain optimum signals by performing Analog-Digital (AD) conversion on received continuous signals at the right time. That is particularly important in order to ensure channel capacity if the sampling frequency of AD conversion is limited.
On the other hand, instead of controlling the timing of AD conversion, it is also possible in digital communication systems to obtain signals at effectively optimum sampling timing by applying digital signal processing such as the interpolation process to samples obtained by asynchronous sampling AD conversion. Such timing optimization in entire digital domains has advantages with respect to downsizing and cost reduction of a receiving apparatus because it becomes unnecessary to equip a control apparatus used for timing control of AD conversion. In order to optimize the timing by means of the digital signal processing of interpolation, it is desirable that the absolute amount of a deviation from an optimum sampling timing in the AD converted signal has been obtained. An example of a timing extracting method, which is the method for deriving such an error between a sampling timing of AD conversion and an ideal timing from a signal obtained by AD conversion, is described in the non patent literature 1.
In the method described in the non patent literature 1, an error in sampling timing is detected by the phase of a clock component whose frequency is equal to the baud rate included in signal intensity. FIG. 16 is a block diagram of a timing error detection apparatus 600 to illustrate the related timing extracting method described in the non patent literature 1. The timing error detection apparatus 600 is provided with an AD converter 610 and a timing extracting unit 620. Here, an input signal is assumed to be a signal received by coherent optical reception after being mixed with local oscillating light.
The received signal is AD converted by the AD converter 610. The AD converter 610 performs samplings four times per symbol of the input signal, that is, it performs quadruple oversampling.
The timing extracting unit 620 is provided with an intensity detection unit 621, a frequency filter unit 622, and a phase detection unit 623. The intensity detection unit 621 receives the signal from the AD converter 610 and detects its intensity by squaring the signal. Next, the frequency filter unit 622 extracts only a frequency component corresponding to the clock frequency from the acquired input signal intensity. Finally, the phase detection unit 623 detects the phase of the extracted clock frequency component and outputs the phase as a timing error signal.
According to the timing extracting method described in the non patent literature 1 as mentioned above, a deviation of sampling timing for an input signal from the optimum sampling timing can be acquired as a measured value directly. Accordingly, it becomes possible to adopt not only a configuration in which timing is optimized by feeding back a timing error signal to an AD converter but also a feed-forward type of optimized configuration in which timing is optimized by digital signal processing at a subsequent stage. Furthermore, the method has an outstanding feature that the timing error signal is not affected by the phase or frequency of a local oscillating light in the coherent optical reception because of utilizing the intensity of an input signal.
On the other hand, an example of another timing extracting method, which is the method for operating with sampling two times per symbol without quadruple oversampling, is described in the non patent literature 2 and the patent literature 1.
The non patent literature 2 discloses a method for calculating a timing error signal for a BPSK (Binary Phase Shift Keying) signal and a QPSK (Quadrature Phase Shift Keying) signal, both of which are generally used in the coherent communication system. However, the timing extracting method described in the non patent literature 2 has a problem that communication quality cannot be secured due to degradation of the performance if the difference in the frequency is large between an optical carrier of a transmission signal and a local oscillating light used at the receiving side.
In contrast, a timing extracting method described in the patent literature 1 is operable even if the difference in the frequency is large between an optical carrier of a transmission signal and a local oscillating light used at the receiving side. However, the sampling timing achieved by feedback control does not become an optimum sampling timing for data decision. Therefore, signal degradation arises, or complication of processing is caused to compensate it.
Furthermore, from the timing error signal obtained by the timing extracting method described in the non patent literature 2 and the patent literature 1, a value of a deviation in timing cannot be directly derived, and only a value which is proportional to the deviation in timing but dependent on signal intensity can be obtained. Accordingly, these methods are not suitable for a feed forward type configuration for timing optimization as the timing extracting method described in non patent literature 1, and a certain amount of time is required for synchronization.
As mentioned above, adopting the timing extracting methods described in the non patent literature 2 and the patent literature 1 to an optical transmitting/receiving method or an optical transmitting/receiving system involves problems.
Patent Literature 1: Japanese patent No. 4303760 (paragraphs [0100]-[0126])
Non Patent Literature 1: M. Oerder and H. Meyr, “Digital Filter and Square Timing Recovery,” IEEE Transactions on Communications Vol. 36, No. 5 1988, pp. 605-612.
Non Patent Literature 2: F. M. Gardner, “A BPSK/QPSK Timing-Error Detector for Sampled Receivers,” IEEE Transactions on Communications Vol. Com-34, No. 5 1986, pp. 423-429.
Non Patent Literature 3: E. de Gabory et al., “DGD Tolerance Enhancement of Optical Polarization Demultiplexing by using Pseudo-Return-to-Zero Modulation Scheme,” Proceedings of the 2009 IEICE Society Conference, The Institute of Electronics, Information and Communication Engineers, Sep. 15, 2009, B-10-85, p. 265.